The high-accuracy measurements of atmospheric CO2
concentration, initiated by Charles David Keeling in 1958,
constitute the master time series documenting the changing
composition of the atmosphere (Keeling, 1961, 1998). These
data have iconic status in climate change science as evidence of
the effect of human activities on the chemical composition of
the global atmosphere (see FAQ 7.1). Keelings measurements
on Mauna Loa in Hawaii provide a true measure of the global
carbon cycle, an effectively continuous record of the burning of
fossil fuel. They also maintain an accuracy and precision that
allow scientists to separate fossil fuel emissions from those due
to the natural annual cycle of the biosphere, demonstrating a
long-term change in the seasonal exchange of CO2 between
the atmosphere, biosphere and ocean. Later observations of
parallel trends in the atmospheric abundances of the 13CO2
isotope (Francey and Farquhar, 1982) and molecular oxygen
(O2) (Keeling and Shertz, 1992; Bender et al., 1996) uniquely
identifi ed this rise in CO2 with fossil fuel burning (Sections 2.3,
7.1 and 7.3).
To place the increase in CO2 abundance since the late
1950s in perspective, and to compare the magnitude of the
anthropogenic increase with natural cycles in the past, a longerterm
record of CO2 and other natural greenhouse gases is
needed. These data came from analysis of the composition of air
enclosed in bubbles in ice cores from Greenland and Antarctica.
The initial measurements demonstrated that CO2 abundances
were signifi cantly lower during the last ice age than over the
last 10 kyr of the Holocene (Delmas et al., 1980; Berner et al.,
1980; Neftel et al., 1982). From 10 kyr before present up to
the year 1750, CO2 abundances stayed within the range 280
± 20 ppm (Indermühle et al., 1999). During the industrial era,
CO2 abundance rose roughly exponentially to 367 ppm in 1999
(Neftel et al., 1985; Etheridge et al., 1996; IPCC, 2001a) and to
379 ppm in 2005 (Section 2.3.1; see also Section 6.4).